When I am sending a mail from my one mail account (Yahoo) to other (Gmail), Gmail automatically puts my mails into the Spam folder. What is the basic criteria to send an email by which mail servers cannot treat my mail as a spam?

+1 I liked the answer, but won't spammers test their proposed spam with the likes of spamcheck.sitesell.com until it complies?
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amelvinAug 17 '10 at 12:50

They are already — spam-detection systems are Bayesian and learn whichever of their hundreds of parameters are most important. For example, exact match of “viagra” in the body text is not so important anymore, because spammers use it rarely, because it was very important before… …shortly, it's an equilibrium system similar to the classic hunter/victim model.
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apostlionAug 17 '10 at 13:05